1. Field of the Invention
The present invention relates to an eddy current damper, and a lithographic apparatus having an eddy current damper.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus contains a plurality of parts where one part is movable relative to another part. Examples of movable parts are a reticle or patterning device support, a wafer or substrate support, a balance mass, etc. Undesired movements of such parts may need to be damped. Such damping may be performed using at least an eddy current damper. However, the use of such an eddy current damper is not limited to lithographic apparatus, and may generally extend to apparatus having parts being movable relative to each other.
Eddy current dampers may use a set of permanent magnets or electromagnets as a source of magnetic field to be coupled to one part of an apparatus, and a body of an electrically conducting material to be coupled to another part of the apparatus, the one part and the other part being movable relative to each other, whereby eddy currents are generated in the electrically conducting body.
As a result of a relative movement of the magnets relative to the electrically conducting body, eddy currents are induced in the body. Consequently, an interaction of the eddy currents and the magnetic field of the permanent magnets generates forces between the magnets and the body that counteract the relative movement. This action is a damping or braking action that is proportional to electrical power produced by the eddy currents, and dissipated in the body. An eddy current damper is applicable both in apparatus with rotary relative movements and in apparatus with linear movements to generate a braking force, or to damp axial or radial vibrations.
The damping of an eddy current damper with a periodic array of magnetic poles is a function of many parameters. When focusing on a damping as a function of a relative movement frequency, it can be observed that the damping decreases significantly at frequencies above a cut-off frequency. The cut-off frequency is determined by a ratio of resistance over inductance of an imaginary coil created in the electrically conducting body of the eddy current damper.
Referring to M. P. Perry “Eddy currents damping due to a linear periodic array of magnetic poles”, IEEE Transactions on Magnetics, Vol. MAG-20, No. 1, January 1984, pages 149-155, if a high damping should be obtained at low frequencies, then a ratio of pole pitch of the magnets over a size of a gap between the magnets and the electrically conducting body should be high. However, the pole pitch is proportional to the coil inductance, and consequently an increase of the pole pitch results in an increase of the inductance and thus a decrease of the cut-off frequency. For this reason, although a high damping may be reached at low frequencies by choosing a large pole pitch, the damping will decrease at relatively low frequencies, and the resulting damper will have a low damping at high frequencies.